The present invention relates to integrated circuits comprising power MOSFETs in parallel with Schottky barrier rectifiers. More particularly, the present invention relates to the integration of trench DMOSFETs and trench Schottky rectifiers upon a single substrate.
Schottky barrier rectifiers (also referred to as Schottky barrier diodes) have been used as synchronous rectifiers in DC-DC power converters. An improved version of a Schottky barrier rectifier is disclosed in U.S. Pat. No. 5,365,102 entitled xe2x80x9cSchottky Barrier Rectifier with MOS Trench.xe2x80x9d A cross-sectional view of such a device is illustrated in FIG. 1. In this figure, rectifier 10 includes a semiconductor substrate 12 of first conductivity type, typically N-type conductivity, having a first face 12a and a second opposing face 12b. The substrate 12 comprises a relatively highly doped cathode region 12c (shown as N+) adjacent the first face 12a. A drift region 12d of first conductivity type (shown as N) extends from the cathode region 12c to the second face 12b. Hence, the doping concentration of the cathode region 12c is greater than that of the drift region 12d. A mesa 14 having a cross-sectional width xe2x80x9cWmxe2x80x9d, defined by opposing sides 14a and 14b, is formed in the drift region 12d. The mesa can be of stripe, rectangular, cylindrical or other similar geometry. Insulating regions 16a and 16b (e.g., SiO2) are also provided on the mesa sides. The rectifier also includes an anode electrode 18 on the insulating regions 16a, 16b. The anode electrode 18 forms a Schottky rectifying contact with the mesa 14. The height of the Schottky barrier formed at the anode electrode/mesa interface is dependent not only on the type of electrode metal and semiconductor (e.g., Si, Ge, GaAs, and SiC) used, but is also dependent on the doping concentration in the mesa 14. A cathode electrode 20 is provided adjacent the cathode region 12c at the first face 12a. The cathode electrode 20 ohmically contacts cathode region 12c. Such a trench MOS Schottky barrier rectifier displays significant improvements in reverse blocking voltage. Typically, two or more individual trench MOS Schottky barrier rectifiers are fabricated in parallel, with rectifiers sharing common anode and cathode contacts. As a result, the individual trench MOS Schottky barrier rectifiers act as a single rectifier.
Unfortunately, Schottky barrier rectifiers, including those described in U.S. Pat. No. 5,365,102, have relatively high on-resistance (forward-biased voltage drop). Moreover, many Schottky barrier rectifiers have relatively high reverse-biased leakage currents. As a result, Schottky barrier rectifiers are frequently replaced for power conversion applications by power MOSFETs (Metal Oxide Semiconductor Field Effect Transistors), which address these problems.
DMOS transistors (Double diffused MOSFETs) (also referred to herein as DMOSFETs) are a type of MOSFET that use diffusion to form the transistor regions. A typical discrete DMOS circuit includes two or more individual DMOS transistor cells which are fabricated in parallel. The individual DMOS transistor cells share a common drain contact (the substrate), while their sources are all shorted together with metal and their gates are shorted together by polysilicon. Thus, even though the discrete DMOS circuit is constructed from a matrix of smaller transistors, it behaves as if it were a single large transistor.
One particular type of DMOS transistor is a xe2x80x9ctrench DMOS transistorxe2x80x9d in which the channel is formed vertically and the gate is formed in a trench extending between the source and drain. The trench, which is lined with a thin oxide layer and filled with polysilicon, allows less constricted current flow and thereby provides lower values of specific on-resistance (forward-biased voltage drop). Examples of trench DMOS transistors are disclosed in U.S. Pat. Nos. 5,072,266, 5,541,425, and 5,866,931.
FIGS. 2a-2C illustrate one embodiment of a conventional trench DMOS structure 120 in which the individual cells 121 are rectangular in shape in a horizontal cross-section. It should be noted that the transistor cells 121 need not have a rectangular shape for basic transistor operation, but more generally may have any polygonal shape. However, a regular rectangular shape and a regular hexagonal shape are generally considered most convenient for layout purposes. The structure includes, in this embodiment, an N+ substrate 100 on which is grown a lightly n-doped epitaxial layer 104. Within doped epitaxial layer 104, a body region 116 of opposite conductivity is provided. An n-doped epitaxial layer 140 that overlies much of the body region 116 serves as the source. A rectangularly shaped trench 124 is provided in the epitaxial layers, which is open at the upper surface of the structure and defines the perimeter of the transistor cell. A gate oxide layer 130 lines the bottom and sidewalls of the trench 124. The trench 124 is filled with polysilicon, i.e., polycrystalline silicon. A drain electrode is connected to the back surface of the semiconductor substrate 100, a source electrode 118 is connected to the source regions 140 and the body region 116, and a gate electrode is connected to the polysilicon that fills the trench 124. As seen in FIG. 2A, the polysilicon lining trenches 124 is continuously connected over the surface of structure 120. In addition, polysilicon contacts 129 extend beyond the surface of structure 120 to serve as interconnects. It should be noted that, rather than having a closed-cell geometry as depicted in the figures, the transistor cell may have an open or stripe geometry.
As indicated, the DMOS transistor shown in FIGS. 2A-C has its gate positioned in a vertically oriented trench. This structure is often called a trench vertical DMOS. It is xe2x80x9cverticalxe2x80x9d because the drain contact appears on the back or underside of the substrate and because the channel flow of current from source to drain is approximately vertical. This minimizes the higher resistance associated with bent or curved current paths or with parasitic field effect construction. The device is also doubly diffused (denoted by the prefix xe2x80x9cDxe2x80x9d) because the source region is diffused into the epitaxial material on top of a portion of the earlier-diffused body region of opposite conductivity type. This structure uses the trench sidewall area for current control by the gate and has a substantially vertical current flow associated with it. As previously mentioned, this device is particularly appropriate for use as a power switching transistor where the current carried through a given transverse silicon area is to be maximized.
Unfortunately, power MOSFETs, including trench DMOS transistors, experience reduced switching speeds due to the long recovery time of the built-in body diode, rendering them less than ideal for high frequency applications.
This problem has been addressed in the art by combining a power MOSFET in parallel with a Schottky barrier rectifier as demonstrated in FIGS. 3A through 3F.
A portion of a prior art trench DMOS transistor is illustrated schematically in FIG. 3A. Such a transistor behaves as if having a built-in body diode Db as shown in FIG. 3A. When provided within a circuit, the transistor shown in FIG. 3A can be illustrated as the portion of FIG. 3B surrounded by the dashed lines. The built-in body diode is represented by D2 in FIG. 3B, which also contains switch S2 associated with the transistor. Also shown in the circuit of FIG. 3B are switch S1 and diode D1, as well as inductor L1, capacitor C1 and load R1 . A voltage Vin is applied across the circuit as shown.
FIG. 3C illustrates two control signals, a first gate drive signal GDS1 for driving switch S1 and a second gate drive signal GDS2 for driving switch S2, at times T1, T2, T3, T4 and T5. As illustrated in FIG. 3B, at time T1, signals GDS1 and GDS2 are such that switch S1 is in the on state and switch S2 is in the off state. As a result, current flows from Vin through the top switch S1, through inductor L1 and through load R1 to ground, as shown by the arrows in FIG. 3B.
Turning to FIG. 3D, at time T2, signals GDS1 and GDS2 are such that both switches S1 and S2 are in the off state. Accordingly, current (as shown by arrows in FIG. 3D) flows through inductor L1 through load R1 and through built-in body diode D2.
Then, as seen in FIG. 3E, at time T3, signals GDS1 and GDS2 are such that switch S1 is in the off state and switch S2 is in the on state. As a result, current (as shown by arrows) flows through inductor L1 through load R1 and through switch S2.
At time T4, signals GDS1 and GDS2 are such that switches S1 and S2 are in the off state, producing a current flow like that shown in FIG. 3D. In particular, current flows through inductor L1 through load R1 and through built-in body diode D2.
Subsequently, at time T5 (and up through T6), signals GDS1 and GDS2 are such that switch S1 is in the on state and switch S2 is in the off state. At this point, the current flow is as shown in FIG. 3B. Specifically, current flows from Vin through the top switch S1, through inductor L1 and through load R1 to ground, as shown by the arrows in FIG. 3B.
It is noted that, if both switch S1 and switch S2 are in the on state simultaneously, then a large current will flow from Vin, through switches S1 and S2, and to ground. In order to avoid this outcome, and as an example, switch S1 is turned off for a brief period before switch S2 is turned on (see, for example, the time period between T2 and T3). During this time period, however, current will pass through the built-in diode D2 as discussed above. For a typical trench DMOS transistor, this current results in a voltage drop across the built-in body diode D2 that is about 0.65V. However, by adding a Schottky diode DS as shown in FIG. 3F, this voltage drop can be reduced to about 0.3V, reducing power dissipation.
Moreover, during the time from T4 to T5, current passes through the built-in body diode D2. At T5, however, the top switch S1 is turned on, and a current passes through the body diode D2 to ground for a certain period of time, due to the long reverse recovery time of the body diode D2. Schottky diodes, on the other hand, have a relatively short reverse recovery time. Hence, if a Schottky diode is provided as shown in FIG. 3F above, much less current will pass to ground, due to this shorter reverse recovery time.
Hence, this arrangement results in a high efficiency system, particularly for high frequency systems requiring low power consumption, such as portable systems. Examples of such systems can be found.in U.S. Pat. No. 5,915,179 entitled xe2x80x9cSemiconductor Device and Method of Manufacturing the Samexe2x80x9d and U.S. Pat. No. 4,811,065 entitled xe2x80x9cPower DMOS Transistor with High Speed Body Diodexe2x80x9d. In U.S. Pat. No. 4,811,065, a Schottky barrier rectifier is combined with a vertical DMOS transistor, within a semiconductor structure, to form a device equivalent to a Schottky barrier rectifier in parallel with an N channel vertical DMOS transistor.
Unfortunately, MOS trench Schottky barrier rectifiers like those disclosed in U.S. Pat. No. 5,365,102, are disadvantageous for integration with a trench DMOS transistors, because the MOS trench of the Schottky barrier rectifier is filled with metal, potentially resulting in a metal coverage problem in the trench area. At present, this difficulty is overcome by integrating trench DMOS transistors and MOS trench Schottky barrier rectifiers together in one package using the MCM (multiple chips module) method, significantly adding to the expense of such devices.
Hence, there is a need in the art to integrate a MOS trench Schottky barrier rectifier with a trench DMOS transistor on a single substrate, without encountering metal coverage problems in the trench area. The present invention meets this need by providing a Schottky barrier rectifier with a polysilicon oxide semiconductor trench, rather than the prior art MOS (metal oxide semiconductor) trench and integrating the same with a trench DMOS transistor on a single substrate.
More particularly, according to an embodiment of the invention, an integrated circuit is provided that has a plurality of trench Schottky barrier rectifiers within one or more rectifier regions and a plurality of trench DMOS transistors within one or more transistor regions. The integrated circuit comprises: (a) a substrate of a first conductivity type; (b) an epitaxial layer of the first conductivity type over the substrate, wherein the epitaxial layer has a lower doping level than the substrate; (c) a plurality of body regions of a second conductivity type within the epitaxial layer in the transistor regions; (d) a plurality of trenches within the epitaxial layer in both the transistor regions and the rectifier regions; (e) a first insulating layer that lines the trenches; (f) a polysilicon conductor within the trenches and overlying the first insulating layer; (g) a plurality of source regions of the first conductivity type within the body regions at a location adjacent to the trenches; (h) a second insulating layer over the doped polysilicon layer in the transistor regions; and (i) an electrode layer over both the transistor regions and the rectifier regions.
An electrode layer is also preferably provided on a surface of the substrate opposing the body regions.
Preferably, the substrate is an n-doped substrate, the first insulating layer is a silicon dioxide layer, and the second insulating layer is a borophosphosilicate glass layer.
According to another embodiment of the invention, an integrated circuit is provided which comprises: (a) a plurality of trench Schottky barrier rectifiers and (b) a plurality of trench DMOS transistors. In this embodiment, the trench Schottky barrier rectifiers and the trench DMOS transistors are integrated upon a common substrate, and trenches associated with the trench Schottky barrier rectifiers and the trench DMOS transistors are filled with polysilicon.
Preferably, the trench Schottky barrier rectifiers and the trench DMOS transistors are fabricated from a common oxide layer and a common polysilicon layer disposed over the oxide layer. More preferably, the trench Schottky barrier rectifiers and the trench DMOS transistors are fabricated using a common substrate, a common epitaxial layer disposed over the substrate, a common oxide layer disposed over the epitaxial layer, and a common polysilicon layer disposed over the oxide layer.
In addition, the anodes of the trench Schottky barrier rectifiers and the sources of the trench DMOS transistors preferably share a common electrode, and the cathodes of the trench Schottky barrier rectifiers and the drains of the trench DMOS transistors preferably share a common electrode.
According to an another embodiment of the present invention, a method is described for manufacturing an integrated circuit comprising a plurality of trench Schottky barrier rectifiers within one or more rectifier regions and a plurality of trench DMOS transistors within one or more transistor regions. The method comprises: (a) providing a substrate of a first conductivity type; (b) forming an epitaxial layer of the first conductivity type over the substrate, wherein the epitaxial layer has a lower doping level than the substrate; (c) forming a one or more body regions of a second conductivity type within the epitaxial layer in the transistor regions; (d) forming a plurality of trenches within the epitaxial layer in both the transistor regions and the rectifier regions; (e) forming a first insulating layer that lines the trenches; (f) forming a polysilicon conductor within the trenches and overlying the first insulating layer; (g) forming a plurality of source regions of the first conductivity type within the body regions adjacent the trenches; (h) forming a second insulating layer over the polysilicon conductor in the transistor regions; and (i) forming an electrode layer over both the transistor regions and the rectifier regions.
In addition, an electrode layer is preferably formed on the surface of the substrate opposite the body regions.
According to several preferred embodiments: (a) the step of forming the body regions includes the steps of forming a patterned masking layer over the epitaxial layer and implanting and diffusing a dopant into the epitaxial layer; (b) the step of forming the trenches includes the step of forming a patterned masking layer over the epitaxial layer and etching the trenches through the masking layer; (c) the step of forming the source regions includes the steps of forming a patterned masking layer and implanting and diffusing a dopant into the body regions; (d) the step of forming the second insulating layer over the polysilicon layer in the transistor region comprises the steps of depositing a BPSG layer over at least the transistor region, forming a patterned masking layer over the BPSG layer, and etching the BPSG layer in areas not covered by the patterned masking layer.
The present invention is advantageous in that it provides a product in which a trench Schottky barrier rectifier is integrated with a trench DMOS transistor on a single substrate and a method for making the same. As a result, such devices can be fabricated with greater ease and less expense than has heretofore been possible.
Still other embodiments and advantages will become readily apparent to those skilled in the art upon review of the Detailed Description, Examples and claims set forth below.